This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in Swiss Patent Application No. 2000 2009/00 having a filing date of Oct. 12, 2000.
1. Technical Field
This invention relates to an apparatus and method for forming sheet products in general and to apparatus and method for forming sheet products that utilize rolls in particular.
2. Background Information
Two-roll rounding machines are known. In these, the sheet-metal blank to be rounded is passed between a rounding roll made of metal and a guide roll with a compressively elastic coating of a plastic material (e.g. Neoprene) with the rolls pressed against each other so that an indented region is formed in the compressively elastic coating, in which deformation of the blank around the rounding roll takes place.
Generally speaking, rounding machines should achieve the best possible rounding of the leading and trailing edge regions of the sheet-metal blank, even in small-diameter rounded sheet-metal tubes with diameters of e.g. 30-80 mm. For this reason, on three-roll rounding machines (e.g., European Patent Applications EP-A 0368686 or EP-A 0096643) the distance between the two working rolls is kept as small as possible. On two-roll rounding machines, flat zone formation is in any event slight owing to the effect of the plastic coating of the guide roll, yet an improvement is desirable here also, particularly when the rounding roll diameter is very small, in the said range of 30 to 80 mm.
Therefore one fundamental problem of the invention is to provide an improved two-roll rounding machine.
According to the present invention, a two-roll rounding machine is provided that includes a first compressively elastic guide roll, a compressively rigid rounding roll, and a second compressively elastic roll. The second compressively elastic roll acts on the compressively rigid rounding roll.
By the provision of an additional roll with a compressively elastic coating (hereinafter called compressively elastic roll for simplicity), further rounding can take place, and this improves the end result. Moreover this additional roll stabilizes the rounding roll by preventing deflection, which is particularly desirable when the diameter of the rounding roll is small.
Preferably, two such additional compressively elastic rolls are provided, yielding, in particular, good rounding and stabilization of the rounding roll against deflection.
Preferably also, one or more back-up rolls are provided which in turn support the compressively elastic rolls, these back-up rolls not normally being rolls with a compressively elastic coating (they may therefore be called compressively rigid rolls).
Preferably also, at least one guide is provided which leads the blank emerging from the first roll pair (guide roll, rounding roll) to the second roll pair (rounding roll, additional roll).
A further fundamental task of the invention is to provide a rounding method which does not possess the said drawbacks.
According further to the present invention, a method for rounding sheet-metal blanks is provided that utilizes a two-roll rounding machine, and wherein the sheet-metal to be rounded is passed between the first compressively elastic guide roll and the compressively rigid rounding roll at least twice. It has been found that particularly good rounding is obtained if the blank makes two passes through the rounding machine according to the invention.